PT149 · S1 · Q17 Discharges of lightning

The author doesn't need to believe that all ash clouds exceed 5 km. They need to exceed that height in order for lightning discharge data to be a viable method of collecting data (since lightning doesn't start discharging until that height). But the author doesn't need to believe that lightning discharge data is always a viable method. He's only claiming that it's sometimes the only viable method. So he just has to assume that, "sometimes when weather radar (and any other means) is not available to measure ash cloud altitude, that ash cloud will exceed an altitude of 5 km".

Nothing in this argument has anything to do with measuring the altitude of regular clouds. The author isn't assuming anything about how weather radar's accuracy compares in different situations.

There's no reason the author needs to assume that most ash clouds that exceed 5 km are at least partly beyond the reach of weather radar. The conclusion is only saying that sometimes lightning discharge data is our only chance at measuring altitude. The author needs to assume that some ash clouds that are beyond the reach of weather radar exceed 5 km (in order to believe that when weather radar is out of reach, lightning discharge data would potentially be an option).

The argument is never talking about small vs. large clouds, so the author hasn't made any assumptions about that sort of comparison. We know that lightning discharges become more frequent the higher the ash cloud moves above 5 km, but that's the height of the cloud, not the size. If we were equating height with size, then we'd say the author's argument more or less contradicts this answer, since the higher ones have more frequent discharges.